1. Field of the Invention
The invention relates to verification of output readings for pressure sensors and more particularly to pressure sensing compound membranes which are deformable both by ambient pressure and electrical activation.
2. Description of the Related Art
Human brain tissue includes gray and white matter suspended in cerebrospinal fluid within the cranium and nourished by blood delivered through cerebral arteries. The gray matter has closely spaced cell bodies of neurons, such as in the cerebral cortex, and the underlying white matter contains densely packed axons that transmit signals to other neurons. Brain tissue has different densities and comprises approximately eighty percent of the intracranial content, with blood and cerebrospinal fluid each normally comprising approximately ten percent.
Cerebrospinal fluid is produced at a rate of approximately 20 ml per hour by secretory cells in several connected chambers known as ventricles and typically is renewed four to five times per day. Cerebrospinal fluid in a healthy human flows slowly and continuously through the ventricles, propelled by pulsations of the cerebral arteries. The fluid then flows around the brain tissues and the spinal column, and through small openings into the arachnoid membrane, which is the middle layer of the meninges surrounding the brain parenchyma and ventricles, where the fluid is finally reabsorbed into the bloodstream.
Under normal conditions, bodily mechanisms compensate for a change in fluid volume within the cranium through tissue resilience and by adjusting the total volume of blood and cerebrospinal fluid so that a small increase in fluid volume does not increase intracranial pressure. Similarly, a healthy brain compensates for an increase in intracranial pressure to minimize a corresponding increase in intracranial volume. This volume- and pressure-relationship can be explained in terms of cerebral compliance, which term is intended to include herein the terms elastance and intracranial compliance.
The brain is compliant as long as a person's auto-regulatory mechanism can compensate for any change in volume. As soon as the brain's auto-regulation or compensatory mechanisms fail, or if too great a trauma to the head occurs, blood and cerebrospinal fluid cannot be displaced, and the brain can no longer adapt to any increase in fluid volume.
A reduction in cerebral compliance eventually will lead to an undesired increase in intracranial pressure, such as described by Seder et al. in “Multimodality Monitoring in Patients with Elevated Intracranial Pressure” from the book “Intensive Care Medicine” published by Springer New York (2008). Reduced cerebral compliance is also referred to as increased brain stiffness or as stiff brain. As more fluid volume is added, a threshold is reached beyond which small increases in volume lead to dramatic and unhealthy increases in intracranial pressure. Intracranial pressure can also increase due to secondary damage to the brain caused by hemorrhage, stroke, infection, tumor, or trauma from sports injuries, automobile accidents or other impacts to the head.
Intracranial pressure has been measured at a number of epi-dural and sub-dural locations, such as described by Steiner et al. in “Monitoring the injured brain: ICP and CBF”, British Journal of Anaesthesia 97(1): 26-38 (2006) and by Brean et al. in “Comparison of Intracranial Pressure Measured Simultaneously Within the Brain Parenchyma and Cerebral Ventricles”, Journal of Clinical Monitoring and Computing 20: 411-414 (2006). Implantable pressure sensors for intracranial pressure monitoring in Intensive Care Units and for long-term monitoring typically are based on deflection of thin membranes. The deflection typically is measured using capacitive or piezoresistive effects.
It is common for piezoelectric sensors such Codman™ MicroSensor pressure sensing probes which use an ASIC based half-bridge strain gauge technology, currently commercially available with ICP Express™ Monitor from Codman & Shurtleff, Inc. of Raynham, Mass., to be calibrated by personnel in the operating room or surgical suite by immersing the sensing probes in saline solution. This pre-implantation immersion corrects for drift which may be induced by water-uptake by polymeric materials used for biocompatible packaging, which can result in swelling effects, or mechanical relaxation effects due to contact with bodily fluids.
Other types of pressure sensors are zeroed during manufacture and the null point is transferred to a patient monitor which may introduce an error and increase the risk of a false pressure reading. Capacitive sensors suffer from drift due to electronic interference with fluids due to stray capacitive effects or changing electrical potentials of surrounding conductive bodily fluids.
Problems with sensor calibration and drift are described in a number of patents including U.S. Pat. Nos. 4,206,762 and 4,281,667 to Cosman, U.S. Pat. No. 4,954,925 to Bullis et al., U.S. Pat. No. 5,361,218 to Tripp et al. and U.S. Pat. No. 5,444,901 to Wiegand et al. An implantable sensor with separate reference circuit is disclosed in U.S. Pat. No. 7,413,547 to Lichtscheidl et al.
Two more recent examples of constructing allegedly self-calibrating pressure sensors, one relative to atmospheric pressure and the other absolute, are provided by Pons et al. in United States Patent Application Pub. No. 2009/0036754. In each example, an actuator is placed at a fixed distance from a deformable membrane of a sensor having at least one piezoelectric transducer, preferably two or four transducers. The sensor further includes a polarization contact such that an electrostatic force is exerted on the membrane when the actuator is charged. In other words, electrostatic actuation is used to deform the membrane in a defined way.
It is therefore desirable to verify the correct functioning of pressure sensors in situ and in an accurate, cost-effective, easy-to-use manner.